Using legacy COM component in C# multithreaded Environment

Question

Hi

I have legacy COM component, and my mission is to write web service that wrap the COM and enable concurrent non-blocking calls.

First of all, because the COM object itself is stateless, i decided to use the [ThreadStatic] attribute so each thread will have its own instance of the COM object to prevent the use of lock { } statement and enable real concurrent processing, but it seems that all calls still procceeded synchronous.

I wrote a test code that runs a method from the COM component synchronous with for { } loop, and then added second thread that doing exactly the same but to another instance of the COM object, and i saw no changes, X calls always consume Y timespan, no matter of threads count. Its like there is a static lock or something...

In spite of that, separate processes can process each one call concurrently for real. What prevent from separate threads to behave the same?

What can i do to enable real concurrent calls to the COM component?

Answer 1

COM is threading aware and will honor the threading model requested by the coclass. It publishes its threading requirements with the ThreadingModel value in the registry. If it is set to "Apartment" (or is missing), COM will make sure all method calls are made from a single threaded apartment by returning a proxy for the interfaces you QI. The proxy ensures the call is marshaled to the correct thread.

You could cheat and use the interface pointer that you got when you created the coclass in an STA thread and make calls without marshaling. Given that the coclass already said it isn't capable of multi-threading, this is very unlikely to work correctly. You'll just randomly corrupt internal state.